72 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 5, 



Besides these main lines, smaller outbreaks occur in the vale of 

 Tremadoc, and may be traced in some of the little hillocks or 

 mounds which form such striking objects in the valley, and which, 

 previously to the erection of the embankment, were islands in the 

 bay. Out of seven of these islets or hillocks, three were found to be 

 composed of dykes of the same character, and apparently of the 

 same age as the more elevated and extended lines forming the 

 boundaries of the valley. In two of these three islands, Yns hir 

 (Long Island), and Yns cerig duon (the Island of the black stone), 

 it occurs in dykes crossing the slates. In the third, Yns cerig aethnen 

 (Island of the shaking stone), it is stratified conformably with the 

 overlying and underlying slates, but has evidently been forced up 

 between the beds of rock, after the solidification of the latter. The 

 slates on both sides are altered to a considerable distance, and the 

 porphyritic rock contains numerous fragments of that formation. 

 The upper beds are decomposing. The entire thickness of the in- 

 trusive rock in this place is about twenty feet. 



In the other islands visited the second class of igneous rocks, 

 comprising beds and dykes of clink-stone, occur. The traces of the 

 igneous origin of these rocks were so striking, that it was with dif- 

 ficulty I could persuade myself that I was not standing on the ruins 

 of an ancient furnace ; and even when I ascertained beyond a doubt 

 the volcanic origin of the igneous matter around me, it was a con- 

 siderable time before I could bring myself to assign to these rocks 

 the very remote origin which repeated observations at last compelled 

 me to do. Like the porphyritic islands, the clink-stone occurs in 

 dykes and also in conformable beds, but the analogy between the 

 two descriptions of igneous rock ceases here. While the effects of 

 the porphyry upon the adjacent beds are only seen upon close in- 

 'spection and comparison of diff'erent portions of the latter, the effect 

 of the clink-stone is of the most marked and striking character. 

 Dark blue slates are turned red or black, are vitrified and calcined 

 for the distance of several feet, and exhibit between their laminae all 

 the hues of the rainbow ; and in the little islet near the farm-house 

 called Pen sy flog, where the clink-stone occurs in a stratified form, 

 there is this important distinction, viz. that while the slate upon 

 which this rock-stone rests is affected, as in the instance of the dykes, 

 and the stratum of slate immediately under and in contact with it 

 is changed into a light ashy substance with white flakes (resembling 

 the ash of inferior kinds of coal), the superincumbent strata are 

 scarcely, if at all aflfected. No fragments of slate-rock were observed 

 in the volcanic matter. 



The igneous rocks of both descriptions, and in the mountain 

 ridges as well as in the islands, are traversed by numerous veins of 

 quartz in large crystals. In a dyke of the clink-stone, about a quarter 

 of a mile from Tremadoc, on the left side of the Old Caernarvon 

 road, in addition to these phaenomena, I observed the edge of a bed 

 of scoriae jutting out on the escarpment and former sea-cliff*. The 

 volcanic cinders, sand, &c. are clearly discernible, as if the result of 

 an irruption of yesterday. I could not observe any important devia- 



